Researchers believe that women with bariatric surgery history should be considered a risk group when pregnant, which means that they should be paid particularly close prenatal attention.

Researchers at Karolinska Institutet examined how bariatric surgery affects pregnancy. The study, which is the most extensive ever done in the field, was based on data from the Swedish Medical Birth Register and the Patient Register.

The study compared over 2,500 babies born between 1992 and 2009 of women who had previously undergone bariatric surgery with 12,500 babies born to mothers who had not. The researchers found that infants of women with bariatric surgery history have lower weights at delivery.

Around 5.2 per cent of them were small for gestational age and were at least two standard deviations below the normal as opposed to only 3.0 per cent in the control group.

Moreover, 4.2 per cent of the babies of mothers with bariatric surgery history were large for gestational age, compared to 7.3 per cent of the control group, and more were born prematurely: 9.7 per cent before the 37th week, compared with 6.1 per cent of the control group. The researchers also found no difference regarding stillbirth or neonatal death (within the first 27 days) between the two groups.

"Mothers with the same BMI gave birth to babies of varying weights depending on whether or not they had undergone bariatric surgery, so there is some kind of association between the two," said Dr Olof Stephansson, obstetrician and Associate Professor at the Clinical Epidemiology Unit at Karolinska Institutet.

"The mechanism behind how surgery influences foetal growth we don't yet know, but we do know that people who have bariatric surgery are at increased risk of micronutrientdeficiencies," said Stephansson.

Researchers also pointed out that bariatric surgery has numerous benefits for mothers, such as lowering the risk of diabetes, cardiovascular disease, cancer and stroke.

Moreover, untreated obesity is a known risk factor for both mother and baby during pregnancy and childbirth.